Carb Question - Do I need to re-jet for higher altitude?

I have a 1973 CJ5 with the original 304 AMC that was recently re-built. It has
a MC 2100 carb with 1.08 mark on it and should have flow around 480 cfm's if my research is correct. I have removed the smog stuff, including the EGR, since it's smog exempt in CA at this point. It's still connected to the manifold vacuum. The jeep tends to have really black soot at the tailpipes and tends to load up when I putter around, and when it loads up, and I get it on the highway and run it up to about 55 MPH, the problem goes away. I live at 4,000
feet and my town is at 3500 feet. I plan to use the jeep at elevations of 3500 feet on up to 8-9000 feet. I pulled off the top of the carburetor today and found that the jets say 47F on them. I spent most of the day researching jets and have read Timgr's posts and understand that at a higher altitude my carb should be running richer, but with the EGR disconnected it should be running leaner. What i'm leaning toward since it's not that difficult to replace the jets is to try some leaner jets, maybe 1 step at a time? Do you trail guys find that you need leaner jets? Also, what kinds of signs should I be looking for if I jet the carb too lean? Can you damage your engine by going too lean? Thanks for reading this long post. Doc

 

I'm in a somewhat similar situation. Run on the road anywhere from 2k ft to 4k ft, wheel mostly from 4k to 8k. All over the map.

Unfortunately for me, I'm running a YF, a pretty expensive Holley rebuild, which is still new enough that it doesn't make financial sense to replace it... the bad part about the YF is that there is no option to simply switch jets, the whole deal is controlled by a stepped needle. I understand that in years gone by, you could obtain a high altitude needle, but no more, and I have been actively looking for years now. My only other option is to make a new needle myself, and it doesn't bother me enough to go that route... yet. I'm afraid the time is coming though, the rich running gets to be a PITA at altitude, especially in summer when its hot.

I guess what I'm trying to say here is yes, were I in your shoes, I would be trying different jets. I'd spend some time studying in depth on how to read plugs first though. There are many tutorials on the subject out there that go well beyond the common "look at the pictures" how-to's out there in googleland, you just have to take the time to dig and study.

 

I think Dave's suggestion about looking at the plugs is a good idea. There are several plug reading charts on the net.

I'd also make sure that the choke is opening completely on this engine. I recall having to block the automatic choke open to get carbureted cars to start above 6000 feet. On a 2100, you can turn the choke plate open by loosening the choke cover screws and turning the black plastic cover. The 2100 uses a thermoelectric choke, and requires both a working hot air connection and a working electric connection. If either is not functioning properly, the choke will not open fully.

Cars from this era are already running fairly lean, in order to minimize hydrocarbon emissions. I would not jump to the conclusion that your altitude is making the mixture too rich. To me, 3500-4000 feet is not that high; certainly not like the 10,000 foot altitudes the CO guys regularly deal with.

If you want to do something about this and don't want to change jets, the later 2150 carb used on the Wagoneer has an altitude compensator built in. No experience with it, but it's said to work well.

hth!

 

My brother lives in Colorado Springs, he advances his timing 10 deg.
Whenever he comes to visit me, he sets it back.

Don't know if this helps, just info.

Jay

 

The original choke on the carb was a unit that connected to the manifold that had no electrical connection, and I replaced this with an electric choke from a MC 2150 that I took off of a 1980 Jeep Wagoneer. I may have had the choke set too rich because it seemed like the engine would run at a fast idle for a long time. How will I know if I have set the choke too lean?

 

There's a spec in the rebuild kit instructions, or in the FSM. Make sure that the choke plate is open fully at operating temperature.

The original '73 2100 choke is thermoelectric. If the 2100 had a choke cover without the lug in the middle for a wire, then it had the wrong choke cover.

Did you reconnect the hot air tube? An '80 Wagoneer should have a thermoelectric choke, and it needs the electrical and the hot air connection. Post a pic if you think that's wrong.

Making the choke too lean won't affect anything other than startup and cold weather driveability during warmup. Once the engine is warmed up, the choke is completely out of the picture, or should be. If you have the choke set too lean, the engine won't start or won't keep running when cold.

 

Thanks for all the help. I think I may have figured out the problem. The choke idle adjustment screw below the choke was screwed too far in and the engine was idling really fast while it warmed up, so I think that is what is causing the tail pipes to be really black. I'll
have to play with that a little to get it right. Tim, you are correct, the original choke on my jeep had a hot water connection, but the one off the Wagoneer must have replaced with an
electric choke only. If you think the thermoelectric one is better, that can go back on real easily.

 

As usual Tim, you are right! The PO plugged the thermo part of the choke that came from the Wagoneer, and the choke cover on the 2100 was replaced with a cover that doesn't have a wire lug. So the question is; can I run that choke as electric only?

 

Hmm. I don't know, but I don't think you can. I expect it will need both hot air and electricity to open fully.

Holley makes an all-electric choke kit. The Motorcraft carbs share a lot of parts and design elements with the Holley aftermarket carburetors. No guarantee, but I'd guess that kit will fit. Some later Fords appear to have an all-electric choke on a Motorcraft carb - check the pick-your-part. Typically these use an oil pressure switch, so that the choke only heats up when the motor is running.

Or you could go with a manual choke kit - that's what I have on my pickup with a 2100.

 

I dont live where this a problem 1000' msl to 3000' msl around here. But i have a question?

In airplanes we have adjustable mixture, and EGT the set to peak. Its there a mod or mods that can be adjusted from the drivers seat?

 

Well, I've learned a lot about carburetors from this thread. I think what I'm going to do is stick with a hot air choke, according to what I read on Pony Carbs website, this works pretty well. Both of my choke housings are junk, so right now I'm searching ebay for a cheap used 2100 or 2150 that has a usable choke housing. If any of you have a parts carb that has a good choke housing that I can buy, let me know. Thanks, Doc

 

When I rebuilt my MC2100 the old choke stove was toast, so I replaced it with a universal style that seems to work pretty good.
Also, even though mine is a thermoelectic choke, I have yet to hook up the electrical connection and have had no issues, the choke opens fully.

http://www.earlycj5.com/forums/showthread.php?t=59575

 

Brieoff, thanks for the post, the bracket for throttle return is a great idea. Where can I find the fuel filter that screws right into the carburetor? A part number would be great!

 

Sorry for taking so long to respond.
Both my NAPA and Autozone have it, I think Wally World does too. Its in the fuel filter section. Don't over tighten, its easy to twist the threaded nipple off the housing. Don't ask me how I know, ugh.

I'm glad you like my master fabricator brackets, they're an advanced design that I pioneered myself.

My Jeep has so much power now that when I drove it the other day the fan puled itself right through my radiator. @#$%#$%#$%
Electric fan time for me.